This invention relates to coherent imaging systems using vibratory energy, such as ultrasonic or electromagnetic waves, and, more particularly to a novel method and apparatus for digital signal processing of the vibratory energy signals.
Various methods and apparatus are already known for imaging object points with vibratory energy, such as ultrasonic mechanical vibrations, and which generally employ at least one array of transducer elements to produce the desired image. In ultrasonic imaging systems, the transducer elements are excited to transmit ultrasonic waves into the object region while further receiving echo signals produced from impingement of the transmitted energy with at least one object point in the object region. One type of such ultrasonic imaging system using a baseband frequency signal processing technique is described in U.S. Pat. No. 4,155,260, which is assigned to the assignee of the present invention, to provide real time imaging that is particularly useful in various analytic arts such as medicine and the like. The disclosed system includes coherent demodulation of the echo signals followed by the time delay and coherent summation of the demodulated signals in a manner dramatically reducing the accuracy necessary for the time delays. In a different imaging system of this general type as disclosed in pending applications Ser. No. 944,482, filed Dec. 19, 1986 now U.S. Pat. No. 4,809,184, and Ser. No. 056,177, filed June 1, 1987 now U.S. Pat. No. 4,839,652, both assigned to the present assignee, the transducer elements are excited with a radio frequency (RF) signal and generate RF analog echo signals for processing in the operatively associated circuitry. All of the above mentioned commonly assigned inventions are specifically incorporated by reference into the present application since the digital signal processing improvements herein disclosed can not only be employed with both type systems but can further improve those ultrasonic imaging systems wherein electrical signals are demodulated to an intermediate frequency (IF) and further processed with associated digital processing circuitry.
As above indicated, certain general considerations apply in forming a real time image with a linear array of transducer elements as now commonly employed especially with respect to the ultrasonic imaging systems being used for medical examinations. Compensation is required therein since the echo from an object point arrives at the receiving transducer elements at different times due to differences in time of propagation of the echo from the object point to the transducer elements. In the phased array sector scanner (PASS) system disclosed in the above referenced U.S. Pat. No. 4,155,260, such compensation is provided with analog signal processing techniques to perform a coherent summation of the echo signals received along the plurality of transducer elements forming the front end of the PASS array. A significant improvement derived with the disclosed ultrasonic imaging system results from signal processing at lower baseband frequencies wherein phase accuracy and time delay accuracy are decoupled from each other and which dramatically reduces the requirements on the circuits used for beam formation. On the other hand, such PASS array remains relatively inflexible, sensitive to minor variations in the operating characteristics of analog circuits, and is relatively expensive (as 2N individual analog demodulation circuits and 2N individual and complete time delay sections are required for an N channel array). Somewhat reduced analog signal processing requirements are involved in the already known ultrasonic imaging system wherein the echo signals are processed at frequencies intermediate between a baseband frequency and RF frequencies. In this type system, a single channel is employed for each individual transducer element wherein the analog echo signal is first demodulated, then band pass filtered, and finally digitally processed for time delay in providing the required coherent summation. The substantial drawbacks encountered with use of analog signal processing techniques in a real time imaging system can be avoided with digital beam formation as disclosed in the above commonly assigned copending patent applications. As therein disclosed, fully digital beam formation is provided in a phased array coherent imaging system, such as an ultrasonic medical imaging system, in a far more accurate, flexible and cost-effective manner. Real-time beam formation is therein achieved with individual channels being provided for the respective transducer elements which convert the analog echo signals to digital data words after preamplification and without converting the RF frequency signals to lower baseband frequencies. As can be further noted in the disclosed digital beam formation, the required coherent summation across individual channels of the array can thereafter be achieved with digital signal processing means to provide a digital representation of the object points detected with the respective transducer elements. Digital baseband processing can be employed to further reduce the complexity of the limited analog circuitry being used in such prior art systems providing a digital beam formation. Understandably, the analog circuit portions of such digital imaging systems must also insure proper signal processing over a wide range of operating conditions.
Still other important considerations apply when any of the aforementioned type imaging systems are used for medical examinations. The dynamic range in such systems should enable detection of all object points in the patient being examined from the scattered echo signals as well as encompass differential losses due to inhomogeneous signal propagation. The largest signals arise from specular reflections which can be illustrated with respect to a human heart organ. Thus, the reflection coefficient for a blood-heart interface is about -30 dB while blood-valve interface can have significantly higher reflection coefficients. Also, the reflection coefficient from the surface of the heart in contact with the thoracic cavity can be nearly 0 dB. Soft tissues echoes arising from volume scattering within the myocardium at the 2 to 5 MHz transducer emission center frequency commonly employed in such type imaging systems can be in the -55 dB range. It follows that dynamic range requirements for the above indicated transmission frequencies to detect the smallest specular echoes as well as the smallest tissue echoes can thereby exexceed 60 dB. A proper medical imaging system should further differentiate between object points exhibiting different echo intensities which reside at a common distance from the transducer array. Such requirement is commonly termed "instantaneous dynamic range" for the particular imaging system and with the requirement relating to object points located on a circular arc having its center at the midpoint of the linear transducer array customarily employed. Such instantaneous dynamic range must account for differential losses due to inhomogeneous signal propagation within the patient or specimen being examined. This problem can be illustrated for a transmitted energy beam traversing soft tissue as compared with traversing a lower loss blood path. The magnitude of such differential loss taking into account the round trip propagation path involved can require a 60 dB instantaneous dynamic range at 3.3 MHz transmission frequency while a 65-70 dB instantaneous dynamic range can be required at a 5.0 MHz frequency operation to ensure that all echo signals of interest will be detected.
All of the aforementioned type imaging systems customarily employ analog-to-digital converter means for the purpose of digitally processing the analog echo signals in order to form the coherent image. For example, such analog-to-digital converter means find use in providing the phase relationship and time delay signals required by the imaging system disclosed in the above referenced U.S. Pat. No. 4,155,260 to do so. Similarly, both imaging systems disclosed in the above referenced commonly assigned copending applications convert the analog echo signal into digital data words for processing with analog-to-digital converter (ADC) means while further recognizing that the sampling ratio involved can require relatively high speed and high cost ADC devices to be employed along with a relatively large amount of high speed memory storage means. As more specifically illustrated in the copending applications, an 8-bit 40 MHz ADC device is employed accompanied by at least 400 words of a high speed RAM for required data delay in each channel of the transducer array. It would be understandably desirable to reduce such requirements with use of standard linear ADC devices but to do so in a manner not degrading the dynamic range of the imaging system. Since the number of bits in the ADC device determines its cost for the most part along with the cost of delay lines operatively associated therewith to digitally process the signals in such type imaging systems it thereby becomes advantageous to reduce the ADC bit requirement without significantly reducing the dynamic range of the modified digital processing means.
Accordingly, it is an object of the present invention to provide a novel coherent imaging system using vibratory energy which digitally processes the analog echo signals in an improved manner.
It is a further object of the present invention to provide a novel coherent imaging system using vibratory energy which includes non-linear processing of the analog echo signals to enable formation of the coherent image with delayed digital data signals.
Still another object of the present invention is to simplify circuit requirements for digitally processing analog echo signals in a coherent imaging system using vibratory energy.
A still further object of the present invention is to provide a novel ultrasonic phased array sector scanner (PASS) to rapidly and accurately sweep a formed ultrasonic beam and which includes non-linear processing of the returned analog echo signals to enable formation of the coherent image with delayed digital data signals.
Another object of the present invention is to provide a fully digital PASS system which digitally processes the analog echo signals in an improved manner.
A further object of the present invention is to provide a novel method of forming a real-time image with vibratory energy which digitally processes the return echo signals in an improved manner.
These and other objects of the present invention will become apparent to those skilled in the art upon reading the following detailed description in conjunction with the appended drawings. In so doing, it should be understood that while the present invention will be described in connection with one energy form, e.g. ultrasonic mechanical vibrations, that still other forms of vibratory energy can be used, such as coherent electromagnetic energy in ladar and radar imaging systems as well as other types of acoustic energy systems such as sonar and the like.